Fixed fallback_language issue

assets/js/src/core/modules/system-settings/components/system-settings-form/components/localization-collapse/components/language-select/language-select.tsx

@@ -35,7 +35,7 @@ export const LanguageSelect = (props: LanguageSelectProps): React.JSX.Element =>
   useEffect(() => {
     const selectedLanguages = alreadySelectedLanguages as string[]
     const newOptions: SelectOption[] = Object.entries(validLocales)
-      .filter(([key]) => !selectedLanguages.includes(key))
+      .filter(([key]) => !selectedLanguages.includes(key.replaceAll('-', '_')))
       .map(([key, value]) => ({
         label: value as string,
         value: key

assets/js/src/core/modules/system-settings/components/system-settings-form/components/localization-collapse/localization-collapse.tsx

@@ -27,11 +27,14 @@ export const LocalizationCollapse = (): React.JSX.Element => {
       return
     }
 
+    const normalizedLanguage = selectedLanguage.replaceAll('-', '_')
     const currentLanguages = (form.getFieldValue(['general', 'valid_languages']) ?? []) as string[]
-    if (!currentLanguages.includes(selectedLanguage)) {
+    if (!currentLanguages.includes(normalizedLanguage)) {
+      const currentFallbacks = (form.getFieldValue(['general', 'fallback_languages']) ?? {}) as Record<string, string[]>
       form.setFieldsValue({
         general: {
-          valid_languages: [...currentLanguages, selectedLanguage]
+          valid_languages: [...currentLanguages, normalizedLanguage],
+          fallback_languages: { ...currentFallbacks, [normalizedLanguage]: [] }
         }
       })
       setSelectedLanguage(undefined)